Control of electrical drives requires determining the position and speed of a rotor. There are several ways to determine such parameters. First, the position of the rotor may be determined by an array of phototransistors and a special shutter coupled to the rotor shaft, or by using Hall-effect sensors. Such systems are described in T. Kenjo, Electrical Motors and Their Controls, Oxford University Press, (1994), 176 pp. Second, the speed signal may be obtained by using a small permanent magnet tachometer generator, attached to the drive, or by using magnetic or optical sensors generating pulses for each angular increment of the rotor. Such systems are described in W. Leonhard, Control of Electrical Drives, 2.sup.nd ed., Springer (1966), 420 pp. Third, a resolver may determine the position of the rotor by a two-phase (sine/cosine) signal at a carrier frequency modulated sinusoidally by the rotation of the rotor. Such a system is described in J. R. Hendershot, Jr. and T. Miller, Design of Brushless Permanent-Magnet Motors, Magna Physics Publishing (1994), p. 1-19. These methods require precise mechanical placement of sensors or mechanical contact between moving parts.
It would be advantageous to form and deliver signals to or from the rotating parts of mechanical or electromechanical devices without either mechanical or galvanic contact and complex sensor supporting systems.
Transferring both the amplitude and the phase of a signal is problematic in an inductive coupling system, because the phase is the primary source of information about angular position of rotor. In existing systems, this phase is obtained by the winding on the rotor placed in the rotating electromagnetic field, formed by the stator windings which are excited by a multiphase voltage. Inductive coupling provides the primary information (phase shift proportional to the angular position of the rotor). However, systems using inductive coupling typically have windings that are difficult to implement by current methods of microelectronics. It is desirable to receive primary information in the form of a phase shift or a linear displacement in a simpler but more "technological" way.